1. Field of the Invention
This invention concerns a chamber, or "oven", in which at least one gas cromatographic column is housed, said chamber being designed to be submitted to a predetermined program of temperature variations in order to obtain a suitable separation of the components of substances which pass through the column and have to be subject to analysis at the column outlet.
Generally speaking, this program of temperature variations includes an initial heating stage, a central stage in which thermal conditions are kept constant or changed according to a preset program, and finally a final cooling stage. The latter temperature may be either higher or lower than the ambient temperature and generally slightly exceeds it; the program, however, changes in function of different factors, among which mainly the nature of the analyzed substance.
2. Description of the Prior Art
To obtain these temperature programs, the gas chromatographic column is housed, as known, in a so called gas chromatographic chamber, essentially consisting of an envelope, for instance parallelepiped-on-shaped with insulated walls, airtight towards the outside so that the internal volume, where the gas chromatographic column in housed, is completely insulated from the outside. In this volume heating means are placed, for instance constituted by one or more resistors, as well as means performing the most possible uniform distribution of the air present inside the envelope and also of heat produced by said resistors, in order to bring all points of the envelope internal volume--and therefore all points of the columns housed in it--at the same preset temperature conditions. The above mentioned air and heat distribution means inside the gas chromatographic oven usually consist of a least one fan suitably positioned inside said close volume and placed in function of the heating resistor position. In this way, satisfactory results are obtained suitable for the operative needs when the chamber is heated, until it reaches the operative temperatures, and also when the chamber is kept at a constant or variable temperature during the central stage of the operative program.
The subsequent final cooling stage, on the contrary, as it is carried out in the gas chromatographic chambers known up to now, results in numerous disadvantages and drawbacks, for which no remedy has yet been found. In fact, in the most general case in which the final cooling temperature is slightly higher than the ambient one, the cooling operation in the known chambers is performed by opening towards the outside the chamber close volume, at least through two air locks which, also with the aid of the fan present within this volume, cause a circulation of external air, cooler than the internal one, until the latter reaches the desired values.
This operative method, necessarily deriving from the structure of the gas chromatographic chambers known up to now, besides involving a relative structural complication for the necessary use of the above mentioned air locks, which must ensure the hermetic closure of the chamber internal envelope and at the same time be insulated like all the chamber envelope walls, causes a first drawback related to the possibility of submitting the oven to thermal shocks risking to damage the chamber itself. In fact, cooling, according to the procedures followed up to now and previously indicated, cannot be performed under the optimal conditions required for this operation and for the others consisting in a temperature change obtained by always keeping at the same temperature all the points of the oven internal envelope and therefore the gas chromatographic column. On the contrary, the opening of said air locks, even if their position is suitably chosen, always involves the formation of preferential currents, which cause zones of lower temperature than the one of the remaining part of the envelope and therefore cause differential cooling in different zones of the gas chromatographic column.
Another drawback, perhaps even more serious than the previous one, consists in the necessity of considering a so called postcooling delay before performing a new cycle of temperature with the same gas chromatographic chamber. In fact, the air cooling inside the oven envelope may be obtained rather quickly using the system of the air locks, but anyway, when the air inside said envelope has reached the desired temperature of cycle end, it is not possible to start a new cycle, as the envelope walls, especially in the insulating layer, still keep heat absorbed during the preceding stage and this heat is gradually transmitted to the internal ambient, which cannot be closed to perform a new operative cycle until heat has not completely disappeared. This involves the necessity of keeping the oven in post-cooling conditions for a considerable time period, during which the chamber, of course, cannot be used.
The above mentioned drawbacks of the known gas chromatographic ovens, which occur when the cooling is carried out by means of ambient air, are even more marked when the cooling is obtained by other means, which should bring the cooling final temperature to lower values than the ambient one. Actually, in these cases, a gas at sub-ambient temperature is for instance introduced, which may even more easily provoke thermal shocks to the gas chromatographic column. Alternatively, it is possible to introduce air cooled by means of a refrigerating system, and in this case, too, there is the possibility of thermal shocks, besides the difficulty of realizing the refrigerating system (which must be a multi-stage one to prevent it may be damaged by excessively hot air treated at the beginning) and the difficulty in the refrigerating system operation, as the air volume treated is relatively high and therefore its cooling gives rise to a considerable condensate.